Passive microwave measurements of sea ice were made at 14,19,31, and 90 GHz in a series of aircraft flights over the Greenland Sea in April 1977. Brightness temperatures and emissivities are computed for four types of ice: miltiyear (MY), first year (FY), young, and new. The results show that (1) emissivity, defined as the ratio of the measured brightness temperature to the physical temperature of the ice surface, is a more fundamental ice parameter and exhibits less sample to sample variation than the brightness temperature; (2) the emisivity of MY ice decreases between 14 and 31 GHz, but changes little or may even increase between 31 and 90 GHz; (3) the emissivity of FY ice is generally 0.09 or greater at all frequencies, except for an occasional decrease at 90 GHz, which may be due to weathering of the FY ice; and (4) the emissivities of young ice and new ice are less than the emissivity of FY ice.